As office automation has advanced and office equipment become more sophisticated, the use of color images has grown accordingly.
In order to meet the growing demand for color imaging, printing apparatuses have been developed that rely on any of several different imaging techniques, such as, for example, inkjet printing methods and electrophotographic methods. Color inkjet printing apparatuses in particular have improved rapidly, using ever-smaller discharged droplets and ever-higher resolutions to provide inexpensive yet high-quality color printing.
However, smaller discharged droplets make it harder to maintain reliable discharge. What happens is that, over time, ink retained in the printhead ink discharge nozzles (hereinafter “nozzles”) increases its viscosity just as, in general, the diameter of the nozzles is being reduced in order to make the discharged droplets smaller, thereby increasing a tendency toward clogged nozzles.
In order to overcome this problem, inkjet printing apparatuses forcefully suck the ink out of the nozzles as convenient or as necessary, to remove clogs or to prevent them from discharge failure. However, since this suction itself wastes a lot of ink, it is necessary to minimize unnecessary ink waste in the suction operation.
Accordingly, in order to reduce the number of such suction recovery operations, several types of detection systems for detecting the presence of ink droplets to be discharged have been developed. Broadly speaking, these systems can be divided into three main types: those that rely on optical detection, those that involve thermal detection and those that use vibration detection. A brief description of each of these techniques follows.
(1) Optical Detection
This technique involves passing the discharged ink droplets through an optical path formed from the light-emitting element to the photosensor, so that the apparatus determines that an ink droplet is being discharged whenever an interruption in the light (that is, the optical path) is detected. A more complete description is disclosed in Japanese Patent Publication For Opposition (KOKOKU) No. 59-6231. A method for using a laser as the light source is disclosed in Japanese Patent Publication Laid-Open (KOKAI) No. 10-119307.
(2) Thermal Detection
This technique includes methods for spraying ink onto a thermo-sensor and detecting an ink discharge from a rise in the temperature of the ink (taught by Japanese Patent Publication Laid-Open No. 54-161938), methods for detecting temperature changes from the vaporization heat at the time the ink is vaporized (taught by Japanese Patent Publication Laid-Open No. 8-323993), and methods for detecting a difference in the trend of a rise in temperature of the printhead during discharge drive when there is ink present and when there is no ink present (taught by Japanese Patent Publication Laid-Open No. 4-6549).
(3) Vibration Detection
This technique includes a method involving striking the ink against an external vibration detection plate and detecting an ink discharge from the vibrations generated from the ink droplets striking the plate (taught by Japanese Patent Publication Laid-Open No. 8-336986) as well as a method for detecting the vibration (shock wave) propagated through the printhead when the ink bubble bursts (taught by Japanese Patent Publication Laid-Open No. 9-201967).
However, the above-described conventional discharged ink droplet detection methods have the following disadvantages:
(1) Optical Detection
With this technique, as the discharged ink droplets are made smaller in order to increase printing density, the proportion of the light occupied by the discharged ink droplets declines compared to the total amount of light reaching the photosensor, resulting in an overall decline in detection sensitivity. As a result, it is increasingly difficult to obtain adequate detection performance in apparatuses employing the smaller discharged ink droplets.
(2) Thermal Detection
With this technique, generally ink containing a non-volatile solution is sprayed directly onto the detection means, so some means is required for removing the ink from the detection means. This requirement complicates the construction of the apparatus.
(3) Vibration Detection
For vibration detection methods involving striking the discharged ink droplets against an external vibration detection plate, there are two main disadvantages. Like the thermal detection technique, this method also requires some means to clean away the ink and so its configuration is somewhat complicated. Moreover, this technique, which involves detecting tiny vibrations propagated inside the printhead when the ink bubbles burst, must be able to suppress all vibration due to sources other than the discharged ink droplets in order to obtain an adequate S/N. However, it is not usually possible to attain such high S/N in the printing apparatuses currently on the market because it is difficult to suppress such extraneous vibrations.
In short, all the conventional discharged ink droplet detection methods currently used in inkjet printing apparatuses described above have some disadvantage. Accordingly, there is an urgent need to be able to detect discharged ink droplets without actually contacting the droplets, and which have the sensitivity to be able to detect and accommodate the higher resolutions and smaller droplet diameters (and smaller overall droplet volumes) that are likely to be developed in the future.
Given the above-described drawbacks of the conventional art, Japanese Patent Application No. 11-100494 taught one such sought-after method, in which electromagnetic waves radiated from the discharged ink droplets are detected without contact during passage of the discharged ink droplets. Since this detection technique is a non-contact type, there is no need for the cleaning means required with the conventional thermal detection method and some of the vibration detection methods described above. Moreover, even a smaller droplet volume is not an impediment to good detection because sensitivity can be improved by raising the temperature of the discharged ink. Therefore it can be said that this detection method is an extremely effective technique for detecting the presence of even the small ink droplets likely to be developed in the inkjet printing apparatuses of the not-so-distant future.
However, the detection method described in Japanese Patent Application No. 11-100494 does have drawbacks of its own. For example, if, during detection, material other than the ink droplets to be detected is present within the detection field or if there is a change in the convection of the air, the amount of energy in the electromagnetic waves present in the detection area will fluctuate, becoming noise during detection and possibly giving false readings. Moreover, even if no such problem as just described actually occurs, detection sensitivity must be improved, by reducing the noise level so as to improve the S/N.